Photovoltaic (“PV”) devices generally consist of one or more active photovoltaic materials capable of generating an electric potential upon exposure to light, and electrical contacts constructed on a suitable substrate that are used to draw off electric current resulting from irradiation of the active PV material. PV devices are generally rigid, either because the active PV material itself is rigid, or because the substrate or other components of the device are inflexible. For example, glass, which is relatively inflexible, is frequently used as a substrate in thin film photovoltaic (“TFPV”) devices because of its strength, durability, tolerance to high processing temperatures and desirable optical properties.
TFPV devices are commonly distinguished from their thicker single-crystal PV counterparts in their ability to absorb light in relatively thin layers, and their ability to function well when fabricated using low-cost deposition techniques, and upon a variety of low-cost, lightweight and flexible substrates. Thus, TFPV devices are being considered for a variety of applications where weight and flexibility are important, such as for space satellites and high-altitude airships.
TFPV devices commonly include a solar absorber layer formed of a Group II-VI material, a Group I-III-VI.sub.2 material, or a Group III-V material. However, a solar absorber layer can be formed of other materials. The term Group II-VI material refers to a compound having a photovoltaic effect that is formed from at least one element from each of groups II and VI of the periodic table. In the context of this disclosure, Group II elements include Zinc, Cadmium, Mercury, and Magnesium and Group VI elements include Sulfur, Selenium, and Tellurium. The term Group I-III-VI.sub.2 material refers to a compound having a photovoltaic effect that is formed of at least one element from each of groups I, III, and VI of the periodic table, where there are two atoms of the group VI element for every one atom of the group I and III elements. In the context of this disclosure, Group I elements include Copper, Silver, and Gold, and Group III elements include Boron, Aluminum, Gallium, Indium, and Thallium. The term Group III-V material refers to a compound having a photovoltaic effect that is formed from at least one element from each of groups III and V of the periodic table. In the context of this disclosure, Group V elements include Nitrogen, Phosphorous, Arsenic, Antimony, and Bismuth.
Prior art TFPV devices with flexible substrates typically use metal foil or polyimide substrates. Metal foil substrates are capable of withstanding the high-temperatures and harsh thin-film deposition conditions needed for the highest efficiency TFPV devices, however, they are relatively heavy and are opaque. The latter characteristic does not allow for bifacial or backside visible light collection from reflected light sources, such as albedo light from either the moon or earth. Nor does this characteristic allow for transmission of undesirable infra-red (“IR”) light through the TFPV device; unused and untransmitted sub-bandgap light increases the operating temperature of the TFPV device and thereby decreases its efficiency. In one example, increased TFPV device operating temperature decreases efficiency by as much as 20%. Finally, opaque substrates do not allow for devices fabricated in the superstrate configuration, where the highest intensity visible light first passes through the substrate. Polyimide substrates are semi-transparent to IR light, however, they are only partially transparent to visible light or capable of withstanding the highest temperature thin-film deposition conditions required for certain CuInGaSe2 (“CIGS”) based devices. Thus polyimide substrates are not suitable for use in superstrate type and bifacial TFPV devices either.
Attempts to provide PV devices on flexible and semi-transparent substrates are disclosed in U.S. Pat. No. 4,816,324, where tetrafluoroethylene-perfluoroalkoxy resin is used as a substrate for the PV device. However, tetrafluoroethylene-perfluoroalkoxy resin cannot withstand processing temperatures higher than 200-250° C., and therefore cannot be used for fabrication of high-efficiency TFPV devices that utilize Group II-VI and Group I-III-VI.sub.2 light-absorber materials, such as Cadmium Telluride (CdTe) and Copper Indium Gallium Di-Selenide (CIGS), respectively, since these materials require significantly higher processing temperatures.
Another example of a substrate that is lightweight, flexible, and that comprises materials such as silicon or silicone resin that are semi-transparent to visible light is described in U.S. Provisional Patent Application Ser. No. 60/792,852, and U.S. Non-Provisional patent application Ser. No. 11/737,119, each of which are incorporated herein by reference. This particular substrate is capable of withstanding processing temperatures up to 600° C., thereby enabling use of high-efficiency CIGS and CdTe materials in fabricating TFPV devices. However, to enable bifacial light collection, both the top and bottom contacts to the TFPV device must be at least semi-transparent to visible light.
To increase efficiency of TFPV devices through bifacial collection, semi-transparent substrates and/or semi-transparent back contacts are needed. For example, a semi-transparent back contact using a thin metal film (e.g., Cu) followed by a transparent conducting oxide (TCO) (e.g., Indium Tin Oxide) has been used with CdTe thin film Group II-VI semiconductor materials grown in a superstrate configuration on heavy, rigid glass substrates, as disclosed in a paper titled “Analysis of a Transparent Cu/ITO Contact and Heat Treatments on CdTe/CdS Solar Cells” by R. Birkmire, S. Hegedus, B. McCandless, J. Phillips and W. Shafarman [Proc. 19th IEEE PVSC (1987), p 967]. However, a thin Cu layer would be difficult to implement with thin-film devices grown in a substrate configuration, because the back contact layer is deposited first and may be damaged and diffuse into other layers during the subsequent processing required for the solar absorber material. Application of the solar absorber material typically includes high heat, vacuum, and use of reactive elements such Se or S. Thus, transparent back contact materials utilized for superstrate configuration cannot necessarily be used for substrate configuration.
Semi-transparent back contact grids have been used along with a highly doped back semiconductor in Group II-VI solar cells materials (e.g., CdTe, ZnTe) in the superstrate configuration on heavy, rigid glass substrates for mechanically stacked four-terminal tandem TFPVs, as disclosed in a paper titled “Polycrystalline CdTe on CuInSe2 Cascaded Solar Cells,” by P. Meyers, C. Liu, L. Russell, V. Ramanathan, R. Birkmire, B. McCandless and J. Phillips [Proc. 20th IEEE PVSC (1988), p1448].
Solar absorbing layers of typical CIGS TFPV devices are p-type and with back contacts formed by intimate connection to thick, opaque metals such as Mo, Ni, or Au that form a low resistance Schottky barrier contact. Thus, these typical high-performance back contacts do not enable visible light to pass through.
It has not been possible to fabricate TFPV devices with semi-transparent back contacts and acceptable performance where the TFPV device is based upon high-bandgap (greater than 1.4 eV) Group I-III-VI.sub.2 materials. For example, when standard semi-transparent TCOs are used without an interface layer as the back contact for wide-bandgap CuInGaSe2 (CIGS) solar absorbing material, low efficiency (e.g., less than 4% efficient) devices result. However, the same back contact layer used with low-bandgap (less than 1.2 eV) CIGS solar absorbing material produces high-efficiency devices (e.g., greater than 10% efficient). Thus, transparent back contact materials utilized for low-bandgap solar absorber materials cannot necessarily be used for wide-bandgap solar absorber materials, as needed for a transparent interconnect in monolithic two-terminal tandem devices.
Monolithic two-terminal tandem devices in the substrate configuration based on crystalline III-V materials and amorphous/microcrystalline Si have been fabricated and commercially sold. In the case of the crystalline III-V bottom cell, the tandem device is fabricated at temperatures greater than 700° C. using expensive deposition equipment for controlled crystal growth that is not amenable to very large area depositions. Thus this technology cannot be reasonably applied to relatively inexpensive large-area depositions using polycrystalline thin-films on low cost and/or flexible substrates. Furthermore, the transparent back contact design concepts of crystalline and amorphous/microcrystalline silicon devices such as tunnel junction interconnects cannot be readily transferred to devices based on polycrystalline CIS and related alloys. Such difficulty in transferring design concepts is due to difficulty in achieving tightly controlled spatial positioning required by tunnel junctions through doping and diffusion of impurities when in the presence of grain boundaries, which act as conduits for diffusion. In addition, very high levels of doping are difficult to achieve in CIS based alloy materials without also creating compensating defects. Thus, other device designs/structures may be preferred.